This invention relates to a vapor phase photoresist silylation process. More particularly, it relates to a low temperature silylation process for use with photoresists having low thermal stability.
The manufacture of microelectronic devices such as semiconductor integrated circuit devices usually involves the patterned etching of resist-patterned layers. Formerly such etching was done with wet chemicals. With the increasing demand for tighter tolerances and higher yields, wet etching has been increasingly replaced by dry etching techniques. Many of these techniques involve the use of oxygen gas as one of the etchant species. The oxygen causes removal or "ashing" of the patterned resist layer and results in unacceptable film thickness loss.
Resists were developed which incorporated silicon-containing groups, typically trimethylsilyl groups. These resists were resistant to oxygen plasma because a silicon oxide barrier formed as etching progressed, but used standard wet development techniques. Incorporation of silicon into the bulk resist was followed by selective incorporation of silicon into either the exposed or unexposed regions of the coated resist. This allowed image development by oxygen plasma and eliminated another wet process step. However, the silylation process itself was a solution process which typically used hexamethylcyclotrisilazane (HMCTS). This produced swelling of the resist images.
Vapor phase silylation with HMDS has been tried and found to be less effective than solution silylation because the high temperatures required cause reflow of many novolac-based resists and cause thermal decomposition of resists such as poly(t-butoxycarbonyloxystyrene) (t-BOC). The high temperatures required are a result of the low reactivity of HMDS.
U.S. Pat. 4,552,833 discloses vapor phase silylation with HMDS of poly(t-butoxycarbonyloxystyrene) (t-BOC) at 85 C for one hour under vacuum. Subsequent studies have shown that images obtained when silylation at such low temperatures is used possess defects which appear to be due to incomplete silylation. Although HMDS can effectively silylate t-BOC at higher temperatures, thermal instability of the resist is a limiting factor. The practical limit for thermal processing of t-BOC is 125-130 C. Above these temperatures, the masking functionality is thermally cleaved from the resist polymer. This causes unselective incorporation of organometallic reagent in both the exposed and unexposed resist regions. As a result, substantial amounts of residuals are obtained after RIE processing. The other reagents disclosed in U.S. Pat. No. 4,552,833 are highly toxic and/or highly corrosive and unsuitable for use in a manufacturing environment.
Although other silicon-containing reagents are theoretically capable of incorporating silicon into a resist film, in a manufacturing environment there are several criteria that must be satisfied. For resists such as t-BOC which have a masking functionality, the silylation temperature must be below the thermal decomposition point of the masking functionality. The silylation temperature must also be below the reflow limit of the resist. Silylation must occur at a rapid rate, so that acceptable throughput levels are achieved, and must go to a high degree of completion, so that a sufficient degree of hardening to RIE conditions can be achieved. Finally, the silylation material should be non-toxic and non-corrosive, and should not introduce metal impurities into the electronic device which can act as dopant materials.